The present invention relates to a method for vulcanizing elastomer products and an apparatus used therefor.
Hitherto, as a method for vulcanizing elastomer articles, such as an automobile tire, with gas as a heat and pressure retaining medium as well as steam as a heating medium using a vulcanizing apparatus with a bladder, there has been employed a method, shown in FIG. 20, wherein an elastomer article (an automobile tire (b) in the example shown in the drawing) is placed in a mold (a), a bladder (c) is inflated by a supply of internal pressure for shaping. After the shape of the tire (b) becomes similar to the inside shape of the mold (a), the mold (a) is closed. Then, steam, as a heating medium, is blown in and supplied from a supply opening (e), which is formed at the center portion of the vulcanizing apparatus and communicates with a supply passage (d), along the horizontal direction from a lower position in the center of the vulcanizing chamber into an inner space (f) of the vulcanizing chamber, so that the tire (b) is heated and pressurized. When the temperature of the tire (b) reaches a prespecified temperature or after a prespecified period of time, the steam supply is stopped and then inert gas such as flue gas, nitrogen gas or the like as a pressurizing medium, at a pressure not lower than (not less than) the steam pressure, is blown in and supplied, until the remaining period in the heating step expires, along the horizontal direction from the same supply opening (e) or along the horizontal direction from another supply opening, which is formed for the pressurizing medium exclusively at the same height as the supply opening (e) for steam and communicates with the same or another supply passage, into the inner space (f) of the vulcanizing chamber, so that the temperature of the tire (b) is maintained at the prespecified temperature by the inert gas.
However, in the above-mentioned prior art apparatus, steam is blown from the lower position in the center of the vulcanizing chamber along the horizontal direction, and accordingly water, due to the condensation of steam, is accumulated in the lower portion of the bottom surface of the tire (b) without being discharged, and as a result, the heating of the lower sidewall is obstructed. An increase of internal pressure causes a decrease of steam inflow, and thereby internal steam flow is weakened. When the internal steam flow velocity is reduced to approximately zero, wet steam forms water droplets and falls downwardly, while other steam which maintains its superheated condition rises upwardly because of its relatively lower specific gravity. This results a temperature gradient along the vertical direction in the tire (b) Further, since the pressurizing gas (inert gas), having a lower temperature than the steam, is blown, the same as the steam, from a nozzle located at a lower position in the tire horizontally toward the lower portion of the tire, the portion to which the gas is directed (i.e. the lower bead portion and the like) is cooled to a lower temperature. Moreover, in a condition wherein the pressurizing gas inflow ceases due to an increase of the internal pressure and accordingly internal flow of the pressurizing gas ceases, the pressurizing gas having a higher specific gravity than steam is apt to accumulate in the lower portion of the inner space (f) of the tire and consequently the temperature of the lower portion, such as the lower sidewall and the lower bead portion, in contact with the pressurizing gas of the lower temperature is lowered.
On the other hand, the remaining steam accumulates in the upper portion of the inner space (f) and is subjected to adiabatic compression, although it only lasts for a short period, because of the pressurizing gas being introduced at a higher pressure, and therefore the steam temperature rises despite the lower temperature of the pressurizing gas and the upper sidewall is heated to a higher temperature.
Thus, in the inner space (f) of the tire, there are formed an upper layer (g) mainly comprising steam, a lower layer (h) mainly comprising the pressurizing gas, and a lowest bottom layer (i) of water of the steam condensate.
Accordingly, the inside temperature of the tire (b) changes as shown by imaginary lines in FIG. 7, FIG. 8, FIG. 14 and FIG. 15. That is, with regard to a point (A1) and a point (A2) at the upper bead portion of the tire , the temperature after the introduction of the pressurizing gas tends to rise as shown by the imaginary line 16, and the imaginary line 116 respectively, in FIG. 7 and FIG. 14, but on the contrary, with regard to a point (B1) and a point (B2) at the lower bead portion of the tire temperature after the introduction of the pressurizing gas tends to fall as shown by the imaginary line 15, and the imaginary line 115 respectively in FIG. 7 and FIG. 14. This results a large temperature difference (T.sub.12) (e.g. 13.degree. C.) between the point (A1) and the point (B1) and a large temperature difference (T.sub.32) (e g. 13.degree. C.) between point (A2) and the point (B2).
With regard to a point (C1) and a point (C2) at the upper sidewall of the tire, the temperature after the introduction of the pressurizing gas tends to rise as shown by the imaginary line 16a and the imaginary line 116a respectively in FIG. 8 and FIG. 14, but on the contrary, with regard to a point (D1) and a point (D2), at the lower sidewall of the tire, the temperature after the introduction of the pressurizing gas tends to stop rising as shown by the imaginary line 15a, and the imaginary line 115a respectively, in FIG. 8 and FIG. 15. This results a large temperature difference (T.sub.22) (e.g. 12.degree. C.) between the point (C1) and the point (D1) and a large temperature difference (T.sub.42) (e.g. 12.degree. C.) between the point (C2) and the point (D2).
Such a large temperature difference, which occurs in the above-mentioned manner, is not rectified completely by the time the vulcanizing operation is finished, and therefore, the upper sidewall and the lower sidewall of the tire (b) will have different degress of vulcanization from each other and this give rise to an undesirable problem in terms of the product quality. In addition, since the necessary cure time is decided according to the lower portions of the tire (b) where the speed of temperature rise is most retarded, a vulcanizing operation of longer duration has to be conducted and this is another undesirable problem also in terms of the productivity and energy saving.
The present invention was made in order to solve the above-mentioned problems, and an object of the present invention is to provide a method and an apparatus for vulcanizing elastomer article which does not cause an undesirable temperature difference inside of the elastomer article during the vulcanizing operation, enables uniform vulcanization to be carried out and accomplishes a reduction of energy loss.